Precipitation interception in Australian tropical rainforests

1. Precipitation interception in Australian tropical rainforests Measurement of stemflow, throughfall and cloud interception D. McJannet, J. Wallace, and P. Reddell Hydrologic Processes 21: 1692-1702

2. Objectives • Measurements • Rainfall • Throughfall • Stemflow • Cloud Interception • Gauge • Canopy water balance • Results • Environmental Importance

3. Rainfall • Average daily rain – 12.1 mm • Tipping bucket gauges • 2 m height • Wind loss correction • Horizontal precipitation inputs

4. Throughfall • Throughfall - Precipitation that is intercepted by canopy that falls to the ground. • 2 throughfall measurements systems per site • 6 m by 100 mm troughs • Collection area of 2.4-3.6 m2 • Leads to a tipping bucket

5. Stemflow • Stemflow - Vegetation-intercepted precipitation that reaches the ground by flowing down the stems or trunks of plants • Large number of different species • Spiral collar of clear vinyl tubing • All trees linked together with a tipping bucket • 12 trees, basal area 1.19 m2 • 13 trees, basal area 1.73 m2.

6. Cloud interception gauge • Cloud Interception - Interception of cloud droplets by the canopy • Collection surface of aluminum screen • Placed under 0.8x0.8 m plywood • Measured through tipping bucket

7. Canopy water balance • Developed equation from one of the rainforest sites • P = 1.14(Sf+Tf)+3.58 • Large storm Ci = {1.14(Sf+Tf)+3.58} – P • Small storm Ci = {(1/Cg)*(Sf+Tf)} – P • P = Precipitation, Sf = Stemflow, Tf = Throughfall, Ci = Cloud interception, Cg = Canopy gap • Closed circles – Ci, Open circles – No Ci • If precipitation falls below the line, Ci occurred

8. Results – Throughfall • Replicated throughfall troughs received similar amounts of water at each site • Throughfall troughs worked well under high precipitation conditions • Throughfall decreased as precipitation event time increased

9. Results – Stemflow • Stemflow was measured 93% of precipitation days • Stemflow was not changed by DBH and species composition • Percentage of rough and smooth bark trees did not alter the results

10. Cloud interception • Gauge interception was increased with wind speed • Gauge interception = 0.41*Estimated Ci • Canopy water balance was a better measure of cloud interception than the interception gauge

11. Rainfall vs Cloud Interception by Month

12. Interception • I = P + Ci – Tf – Sf • Interception rates are high for these lower montane forest sites • ~1000 mm of precipitation evaporation annually

13. Environmental Importance • Throughfall and stemflow sampling worked well in the remote sites tested • Even under intense periods of rain • Monthly servicing • Under more variable canopies, more throughfall gauges may be necessary • Can give a good idea about the canopy water balance of a site

14. Environmental Importance • Water balance vs cloud interception gauge • Canopy water balance determined Ci better than a Ci gauge • Gauge does a good job of assessing presence of cloud interception • Small storms can show no Ci through canopy water balance • All Ci held in the canopy

15. Environmental Importance • Cloud interception can be a large contribution to the water budget of a tropical rainforest • Up to 70% of total throughfall and stemflow • Important source of water during the dry season

16. Questions?